1. Field of the Invention
This invention pertains to the art of load balancing control of multiple centrifugal compressors.
2. Description of the Prior Art
U.S. Pat. No. 3,648,479 (assigned to the assignee of this application) discloses a load balancing control arrangement for a multiple centrifugal compressor refrigeration system. Our invention is considered to be an improvement in the specific circuitry of that arrangement in that it will provide improved load balancing in refrigeration systems having at least three compressors.
In that patent, the "Background of the Invention," which is equally applicable here, is expressed as follows:
It is known that an electric motor-driven centrifugal gas compressor is capable of overloading the motor if allowed to seek loads higher than the standard designed conditions. Centrifugal gas compressors, when used to compress a refrigerant gas, are therefore required to have variable capacity controls and one such control determines the positions of movable suction inlet prerotation vanes for the compressor to vary the compressor capacity. An electrical control circuit for automatically positioning such inlet vanes to provide a predetermined control of refrigeration or air conditioning system parameters, such as for example the chilled outlet water temperature, is disclosed in the then pending, now issued, U.S. Pat. No. 3,609,989. As there disclosed, a current sensing coil is also provided to operate an overload relay whenever the current drawn by the electric motor driving the compressor exceeds a predetermined amount corresponding to an overload condition. The operation of the overload relay causes the inlet vanes of the associated compressor to move in a decreased capacity direction until the sensed overload current is reduced to a safe value at which the operated overload relay restores to the nonoperated condition upon which the movement of the inlet vanes toward a decreased capacity position is stopped. It is sometimes desirable to have two or more centrifugal gas compressors connected to the same common refrigerant load circuit and each gas compressor should have its own capacity control circuit corresponding to the common system condition, such as chilled water temperature or the like. A problem exists with such an arrangement in that one of the compressors tends to work harder than the other compressors in the system; and therefore it is desirable to provide a load balancing capacity control to evenly divide the common load among all of the operating compressors. Such a control should be simple, reliable and also inexpensive.
As noted, that expression of the background is equally applicable here.
There are several deficiencies in the specific circuitry illustrated in the noted U.S. Pat. No. 3,648,479. These deficiencies will be taken up in connection with FIGS. 1 and 2 of this application which correspond to the prior art FIGS. 1 and 2 of the noted U.S. Pat. No. 3,648,479.